In a line printer, one of a plurality of print hammers is selectively driven by a corresponding electromagnet to press an ink ribbon and one or more sheets of paper on a desired type which is disposed on a type carrier, such as a belt, arranged to move around a drive pulley and an idler pulley at high speed. In order to determine the print timing, one of character marks disposed on the type carrier is detected by means of a pickup, and then a corresponding electromagnet is energized. The interval between the time of energization of the electromagnet and the time of actual impact, i.e. the time of printing, is referred to as flight time. The flight time is apt to vary due to various reasons, such as fluctuation of the driving voltage applied to the electromagnet. If the print timing deviates excessively from a desired value, the hammer face of each print hammer cannot cover the entire type face resulting in imperfect printing. Namely, with such incorrect timing characters are printed whose right or left side portions are broken off.
In accordance with a conventional technique, a means for adjusting the print timing is employed, in which the time constant of a monostable multivibrator is adjusted by means of a variable resistor or the like so as to compensate for the variation in flight time. With such a means for adjusting print timing, however, the timing cannot necessarily be set to the most suitable or ideal value with which imperfect printing is prevented even though the flight time varies within a tolerance of flight time variation.Therefore, according to the conventional technique imperfect printing is apt to occur even when the flight time varies only within a tolerance, and thus the print timing has to be readjusted each time such imperfect printing occurs.